An infrastructure-based wireless network typically includes a communication network with fixed and wired gateways. Many infrastructure-based wireless networks employ a mobile unit which communicates with a fixed base station or access point that is coupled to a wired network. The mobile unit can move geographically while it is communicating over a wireless link to the base station or access point. When the mobile unit moves out of range of one base station or access point, it may connect or “handover” to a new base station or access point and starts communicating with the wired network through the new base station or access point.
In comparison to infrastructure-based wireless networks, an ad hoc network typically includes a number of geographically-distributed, potentially mobile units, sometimes referred to as “nodes,” which are wirelessly connected to each other by one or more links (e.g., radio frequency communication channels). The nodes can communicate with each other over a wireless media without the support of an infrastructure-based or wired network. Links or connections between these nodes can change dynamically in an arbitrary manner as existing nodes move within the ad hoc network, as new nodes join or enter the ad hoc network, or as existing nodes leave or exit the ad hoc network. Because the topology of an ad hoc network can change significantly techniques are needed which can allow the ad hoc network to dynamically adjust to these changes. Due to the lack of a central controller, many network-controlling functions can be distributed among the nodes such that the nodes can self-organize and reconfigure in response to topology changes.
One characteristic of the nodes is that each node can directly communicate over a short range with nodes which are a single “hop” away. Such nodes are sometimes referred to as “neighbor nodes.” When a node transmits packets to a destination node and the nodes are separated by more than one hop (e.g., the distance between two nodes exceeds the radio transmission range of the nodes, or a physical barrier is present between the nodes), the packets can be relayed via intermediate nodes (“multihopping”) until the packets reach the destination node. As used herein, the term “multihop network” refers to any type of wireless network which employs routing protocols among nodes which are part of a network. In such situations, each intermediate node routes the packets (e.g., data and control information) to the next node along the route, until the packets reach their final destination. Nodes in the mesh network use end-to-end path metrics to select a path, from the multiple path options to any destination. The path metrics are generally sum of the individual link metrics along the path.
Multiple-Input Multiple-Output (MIMO) technology and IEEE 802.11n
Today Multiple-Input Multiple-Output (MIMO) technology is being used in many next generation wireless products. IEEE 802.11n specifies the use of MIMO technology for wireless local area networks or WLANs. IEEE 802.11n is a proposed amendment to the IEEE 802.11-2007 wireless networking standard that seeks to significantly improve network throughput over previous standards, such as 802.11b and 802.11g. Draft 7.0 of the IEEE 802.11n standard was approved in November 2008. As used herein, “IEEE 802.11” refers to a set of IEEE Wireless Local Area Network (WLAN) standards that govern wireless networking transmission methods. IEEE 802.11 standards have been and are currently being developed by working group 11 of the IEEE Local Area Network (LAN)/Metropolitan Area Network (MAN) Standards Committee (IEEE 802). Any of the IEEE standards or specifications referred to herein may be obtained at http://standards.ieee.org/getieee802/index.html or by contacting the IEEE at IEEE, 445 Hoes Lane, PO Box 1331, Piscataway, N.J. 08855-1331, USA.
IEEE 802.11n builds on previous 802.11 standards by adding multiple-input multiple-output (MIMO), and frame aggregation at the MAC layer.
MIMO technology relies on multipath signals—reflected versions of the same signal that arrive at the receiver at different times since they travel over different paths. MIMO uses the multipath signal's diversity to increase a receiver's ability to recover the message information from the signal.
MIMO technology also utilizes Spatial Division Multiplexing (SDM). SDM is the method of transmitting multiple spatial streams from each of the multiple antennas. Each spatial stream is independent of each other and separately encoded. SDM spatially multiplexes multiple independent data streams, transferred simultaneously within one spectral channel of bandwidth. By transmitting different spatial streams of symbols in parallel the throughput or transmission data rate can be increased. Each spatial stream requires a discrete antenna at both the transmitter and the receiver. In one implementation, the IEEE 802.11n draft standard proposes transmitting packets simultaneously using two different bandwidths, and up to four different spatial streams. This can greatly improve performance.
Although MIMO technology increases the physical transmission data rate this does not directly translate into higher throughput. To achieve high throughput IEEE 802.11n proposes use of frame aggregation techniques. Frame aggregation refers to combining many small frames into a larger “aggregate” frame and then transmitting the larger aggregate frame to increase throughput of the network. Multiple packets can be easily aggregated to form large frames that significantly increase the overall throughput. The large aggregate frame has significantly lower Media Access Control (MAC) overhead in comparison to the smaller non-aggregated frames. At the same time, although sending large aggregate frames increases the throughput, packet loss associated with these big aggregate frames can also lead to large drops in throughput.
Thus, coupling MIMO architecture with wider bandwidth channels offers the opportunity of creating very powerful yet cost-effective approaches for a significant increase in the maximum raw (PHY) data rate.
As described above, multiple streams are used to attempt to increase throughput. To transmit on multiple streams, multiple radio frequency (RF) channels are required, and this can only be achieved when there is sufficient antenna separation and sufficient multipath is available in the channel. If the transmitter attempts to send packets on multiple streams when multiple streams are not actually available, packets can be lost. In other words, transmission of packets in parallel requires the availability of non-correlated channels. Inadvertent transmission of packets on multiple streams when channel is correlated can result in packet reception failures at the receiver since the receiver cannot decode the packet. As a result, the entire packet must be retransmitted, and the retransmission of a large packet results in a significant drop in achievable throughput. Retransmissions of large aggregate frames can significantly decrease MAC efficiency and overall throughput. As such, an intelligent system is needed to avoid retransmission of such frames.
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